U.S. patent number 5,287,543 [Application Number 07/772,207] was granted by the patent office on 1994-02-15 for multichannel communication system with an amplifier in each channel.
This patent grant is currently assigned to General Electric Co.. Invention is credited to Herbert J. Wolkstein.
United States Patent |
5,287,543 |
Wolkstein |
February 15, 1994 |
Multichannel communication system with an amplifier in each
channel
Abstract
A multichannel communication system carries signals having a
different frequency range in each channel. In order to achieve a
particular output power level from each channel, an amplifier is
associated with each channel to boost the signal level. For
reliability, a switching arrangement switches amplifiers among the
channels in accordance with a priority, or substitutes a redundant
amplifier for a degraded or failed unit in a channel. The amplifier
is subject to distortion at the desired output level, and is
cascaded with a distortion equalizer or linearizer for reducing the
total distortion. In accordance with the invention, each distortion
equalizer is fixedly connected in one channel, and is optimized for
the relatively narrow frequency range of that channel, rather than
being switched together with the amplifier and being optimized over
the total or cumulative bandwidth of all the channels.
Inventors: |
Wolkstein; Herbert J.
(Livingston, NJ) |
Assignee: |
General Electric Co. (East
Windsor, NJ)
|
Family
ID: |
25094290 |
Appl.
No.: |
07/772,207 |
Filed: |
October 7, 1991 |
Current U.S.
Class: |
455/13.3;
330/124R; 342/354; 455/103; 455/20; 455/8 |
Current CPC
Class: |
H04B
7/2045 (20130101) |
Current International
Class: |
H04B
7/204 (20060101); H04B 007/185 () |
Field of
Search: |
;455/8,12.1,13.3,13.4,20,101,103 ;342/353,354 ;370/75 ;333/105
;330/124R,124D,126 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Pham; Chi
Attorney, Agent or Firm: Meise; W. H. Berard; C. A. Young;
S. A.
Claims
What is claimed is:
1. A multichannel amplifying arrangement including a multiplicity
of channels, said arrangement comprising:
a plurality N of sources of signal, each of said sources of signal
being operative at a different frequency range, said different
frequency ranges cumulatively covering a predetermined total
bandwidth, each of said sources of signal being coupled to the
input of one of said channels;
a plurality of at least N power amplifiers, each including an input
port and an output port, and each for amplifying signal applied to
said input port and for generating amplified signal at said output
port, each of said power amplifiers having a bandwidth
substantially equal to said total bandwidth, and each of said power
amplifiers being subject to nonlinearities at certain operating
levels;
utilization means coupled to an output port of each of said
channels for using amplified signal therefrom;
controllably switchable interconnection means coupled to said input
and output ports of each of said plurality of power amplifiers, and
by way of said channels to said N sources and to said utilization
means, for coupling each of said power amplifiers in one of said
channels, with any of said plurality of power amplifiers which are
in excess of the number of active channels being held in reserve,
and in case of degradation of one of said power amplifiers
associated with a particular channel, for selectively switching
said degraded power amplifier out of said particular channel, and
for switching into said particular channel an operational one of
said power amplifiers, the bandwidth of which includes the
frequency range of said particular channel, whereby said
operational one of said power amplifiers amplifies signals in said
particular channel; and
a plurality N of linearization means adapted for compensating said
nonlinearities, each of said linearization means being fixedly
associated with one of said channels and switchably associated with
that one of said power amplifiers coupled by said switchable
interconnection means in said one of said channels, whereby each of
said linearization means must have a bandwidth sufficient to cover
only one of said frequency ranges, and not said total
bandwidth.
2. An arrangement according to claim 1, wherein said N sources
comprise:
antenna means for simultaneously receiving a plurality of said
signals, each in one of said different frequency ranges, and for
coupling said signals to a single transmission path; and
frequency separation filter means including an input port coupled
to said transmission path, and also including a plurality of output
ports, for frequency separating said signals into said plurality of
channels.
3. An arrangement according to claim 2, further comprising
frequency conversion means coupled in said transmission path,
between said antenna means and said input port of said frequency
separation filter means, for converting each of said signals
received by said antenna means to another frequency.
4. An arrangement according to claim 1, wherein each of said power
amplifiers comprises a travelling-wave tube.
5. An arrangement according to claim 1, wherein each of said
linearization means comprises a predistortion equalizer; and
each of said predistortion equalizers is coupled in one of said
channels at a location between the corresponding one of said
sources and the associated portion of said switchable
interconnection means.
6. A spacecraft comprising:
a polarized receiving antenna for receiving N signals over a first
frequency range in a first polarization to produce received
signals;
a frequency converter coupled to said antenna for converting said
received signals to a second frequency range, within which second
frequency range each of said signals exclusively occupies a
predetermined portion;
a frequency separation filter coupled to said frequency converter,
for separating said signals from each other depending upon which
portion of said second frequency range said signal occupies, to
thereby separate said signals into N separate channels;
a transmitting antenna;
a signal combiner coupled to an output of each of said channels and
to said transmitting antenna for combining signals received from
each of said channels to produce a combined signal, and for
coupling said combined signal to said transmitting antenna for
transmission;
a plurality at least equal to N of amplifiers, each of said
amplifiers including an input port for receiving one of said
signals and an output port at which amplified signal is produced,
each of said amplifiers being subject to nonlinearities which
depend upon signal level;
a switching arrangement coupled to each of said N channels, and
also coupled to said input and output ports of said amplifiers, for
coupling one of said amplifiers into each of said channels, and
for, in any of said channels, replacing that one of said amplifiers
currently in-circuit with another one of said amplifiers; and
a plurality N of distortion correctors, each of said distortion
correctors being fixedly coupled in one of said channels, for
correcting the distortion of that one of said amplifiers currently
operating in said one of said channels.
7. A spacecraft according to claim 6, wherein each of said
distortion correctors comprises a predistortion correction means,
each including an input port and an output port, said input ports
of each of said predistortion correction means being fixedly
coupled to a different output of said frequency separation filter,
and said output ports of each of said predistortion correction
means being coupled by way of a portion of said switching
arrangement to said input port of one of said amplifiers.
8. A spacecraft according to claim 7, further comprising a
plurality of driver amplifiers, each of said driver amplifiers
being fixedly coupled in one of said channels between one of said
outputs of said frequency separation filter and the associated one
of said predistortion correction means.
9. A method for operating a spacecraft communication system,
comprising the steps of:
receiving a plurality of transmitted information signals to be
retransmitted;
converting the carrier frequency of said information signals to
produce converted signals;
separating said converted signals, in accordance with frequency,
into a plurality of separated signals, each in a different channel,
each channel having a different channel bandwidth, the cumulative
channel bandwidth defining a total bandwidth greater than that of
any one of said channel bandwidths;
combining the signals at the outputs of said channels to produce a
combined output signal;
applying said combined output signal to a transmitting antenna for
retransmission;
within at least one of said channels, switchably coupling, for
operation within said one of said channels, an operable power
amplifier having at least said total bandwidth, for amplification
of said separated signal in said one of said channels, for
producing an amplified signal subject to nonlinearity at some
operating levels;
within at least said one of said channels, correcting said
nonlinearity by means of a linearizer dedicated to said channel and
not to any one of said amplifiers.
10. A method according to claim 9, wherein said step of correcting
said nonlinearity includes the step of predistortion each of said
separated signals before said amplification.
Description
BACKGROUND OF THE INVENTION
This invention relates to multichannel communication systems
including power amplifiers, and more particularly to distortion
correction in a multichannel system.
Modern communication satellites provide several broadband repeater
channels, which receive from an Earth station a plurality of
signals within a cumulative frequency band, process the received
signals, as by low-noise amplification, filtration and block
conversion to another frequency, for retransmission to the same or
another location. While the description of the invention is couched
in terms of a communications satellite, the invention may also be
used for terrestrial or other uses.
FIG. 1a illustrates, in simplified block diagram form, a satellite
body 6 in accordance with the prior art, upon which are mounted a
polarizing grid arrangement 8, vertically polarized receiving
antenna 12V and horizontally polarized receiving antenna 12H.
Receiving antennas 12V and 12H are coupled to vertical and
horizontal signal processing arrangements 10V and 10H,
respectively, located within body 6. Signal processing arrangements
10V and 10H process the received signals to produce signals to be
retransmitted, which are broadcast by transmitting antennas 32V and
32H, respectively. Signal processing arrangement 10H is similar to
vertical processing unit 10V, so only processing unit 10V is
described.
The nature of the signals arriving at the satellite may be
understood by reference to FIG. 1b. The vertically-polarized
signals arriving at antenna 12V by way of polarizing grid 8
includes a plurality of signals centered at different frequencies
f1, f2, f3. The amplitude spectra of various signals are designated
V1, V2, V3 in FIG. 1b. Some of the signals arriving with horizontal
polarization (at antenna 12H) are illustrated (in dashed lines) as
H1, H2 in FIG. 1b. In a typical satellite system, there may be as
many as 10 or more vertical (V) and 10 or more horizontal (H)
channels, with their frequencies of operation interleaved as shown
in FIG. 1b. The bandwidth of a signal such as signal V2 may be
sufficient to carry a television channel, or more. Thus, the
bandwidth of a signal such as V2 may be 6 MHz or more. Vertical
processing channel 10V of FIG. 1a may, as a consequence, receive 10
or more signals V1, V2, V3 . . . V.sub.N, each six or more MHz
wide, which are separated from each other by a like amount. Thus,
the total frequency bandwidth occupied by the vertical signals may
be 120 MHz or more, calculated as [10(v)+10(H)].times.6. The center
frequency of the 120 MHz band may be, for example, at 14 GHz.
The 10 or more vertical signals V1, V2 . . . received by antenna
12V of FIG. 1a are coupled to an input filter 14 of channel 10V,
for reducing noise and preventing interference. Filter 14 is a
bandpass filter with a bandwidth substantially equal to the total
bandwidth of the vertical signals. The filtered signals are coupled
from input filter 14 to a low noise amplifier (not illustrated) if
required, and then to a block converter including a mixer 16 and a
local oscillator 18. The frequency of local oscillator 18 is
selected to convert the 14 GHz center frequency to some other
center frequency, such as 12 GHz. The downconverted 12 GHz signals
are applied over a transmission path 20 to a multiplexing (MUX)
filter 22. Multiplexing filter 22 separates signals V1, V2, V3 . .
. from each other in accordance with their frequencies.
Multiplexing filter 22 is the starting point for a plurality of
separate channels designated generally as 1, 2, . . . 3, 4. If
there are 10 vertical signals V1, V2, V3 . . . , then the number of
channels in signal processor 10V is also 10. The signal in each of
channels 1, 2, . . . 3, 4 is one of the signals V1, V2, . . . . In
effect, filter 22 is a source of signals at a plurality of
different frequencies, driving a like plurality of separate
channels.
In general, the signals on channels 1, 2, . . . 3, 4 in FIG. 1a are
amplified, the distortion generated due to the amplification is
compensated, and the amplified and distortion corrected signals are
applied to a combiner or demultiplexer 30, which may be a filter
similar to filter 22 operated in reverse, or it might be a group of
hybrid combiners which do not discriminate based upon frequency.
The combined signals at the output of combiner 30 are applied to a
transmitting antenna 32V for transmission back to an Earth station,
or possibly to another satellite.
System considerations such as the signal strength of the signal
available at the satellite, the receiving antenna gain, and the
transmitting antenna gain and field strength required to reach the
ground station establish the overall power gain which must be
provided in each channel between receiving antenna 12V and
transmitting antenna 32V.
Within any channel 1, 2, . . . 3, 4 of FIG. 1a, the signal is
processed by the cascade of a driver amplifier (DA) 34, a
distortion linearizer such as a predistortion equalizer (PDL) 36,
and a power amplifier or final amplifier (FA) 38. For example, as
illustrated in FIG. 1a, the cascade of a DA 34.sup.1, PDL 36.sup.1
and FA 38.sup.1 processes the channel 1 signals, and a DA 34.sup.2,
PDL 36.sup.2, and FA 38.sup.2 amplifies the signals for channel 2.
As illustrated in FIG. 1a, an additional cascade of a DA 34.sup.5,
PDL 36.sup.5, and FA 38.sup.5 is connected in cascade, to define a
supernumerary "channel" designated 5. Channel 5 is not connected
for handling signal, but instead represents a reserve cascade which
may be substituted into any of the other channels in which the
cascade may become defective. To this end, connection between input
filter 22 and the inputs of the various channel cascades 34, 36, 38
is provided by means of an input switch arrangement designated 24,
and connection between the outputs of final amplifiers 38 and
combiner 30 is provided by an output switch arrangement designated
as 28. A switch control arrangement illustrated as 26 gangs the
input and output switches for simultaneous operation, and responds
to signals in response to evidence of failure, generated on the
ground or autonomously by control circuits within the spacecraft
itself. Thus, in the event that the cascade of DA 34.sup.1, PDL
36.sup.1 1, and FA 38.sup.1 fails completely or becomes degraded,
the reserve cascade including DA 34.sup.5, PDL 36.sup.5, and FA
38.sup.5 can be substituted therefor, with the cascade of DA
34.sup.1, PDL 36.sup.1, and FA 38.sup.1 being removed from on-line
use. Naturally, additional redundant units may be provided, and if
the number of failures should exceed the number of redundant units,
the switching arrangement including 24, 26 and 28 may move operable
cascades from lower-priority uses to higher-priority uses. In order
to be switchable to obtain this level of reliability, each cascade
must have an instantaneous frequency bandwidth covering the
cumulative or total bandwidth of the vertical signals V1, V2, V3, .
. . .
An improved communications system is desired.
SUMMARY OF THE INVENTION
Recognizing that broadband final amplifiers of the solid-state
type, and even those using modern traveling-wave tubes (TWTs), have
very similar distortion characteristics among themselves, the
switching arrangement is repositioned so that the distortion
equalizers are fixedly associated with the individual channel,
rather than being switchable together with the amplifier. As a
result, the distortion equalizer may be designed and optimized
during manufacture for the relatively narrow bandwidth of the
channel, rather than for the total bandwidth of all the
channels.
DESCRIPTION OF THE DRAWING
FIGS. 1a and 1b is a simplified block diagram of a prior art
spacecraft communication systems, and FIG. 1b illustrates a
simplified portion of an amplitude-frequency spectrum associated
with the arrangement of FIG. 1a;
FIGS. 2a and 2b is a simplified block diagram which illustrates a
portion of a spacecraft communication system in accordance with the
invention, and FIG. 2b is a portion thereof; and
FIG. 3 illustrates a portion of a spacecraft communication system
according to another aspect of the invention.
DESCRIPTION OF THE INVENTION
FIG. 2a is a simplified block diagram of a portion of a spacecraft
communication system in accordance with the invention. Elements of
FIG. 2a corresponding to those of FIG. 1a are designated by the
same reference numerals. In FIG. 2, each cascade of a driver
amplifier 34, predistortion equalizer 36, and final amplifier 38 is
redistributed so that the driver amplifier and predistortion
limiter are fixedly associated with each channel, between the
multiplexing filter 22 (the effective input of the channel) and the
input of switching arrangement 24. Thus, driver amplifier 34.sub.1
and predistortion linearizer 36.sub.1 are cascaded between the
channel 1 output of multiplexing filter 22 and input switch 24. As
illustrated in FIG. 2a, switch arrangement 23 connects final
amplifier 38.sub.1 in channel 1. The net gain in channel 1 between
the channel 1 output of multiplex filter 22 and the channel 1 input
of combiner 30 is identically the same as in the arrangement of
FIG. 1a (assuming, of course, that the elements themselves are
identical). Similarly, the gains through each of the channels of
FIG. 2a are the same as in FIG. 1a. However, only the final
amplifiers 38 are required to have the cumulative bandwidth of all
the vertical-polarization channels, if the redundancy scheme so
requires, while the driver amplifiers and predistortion linearizers
require only the relatively narrow channel bandwidth. For the
previous example of 10 vertical channels, each with 6 MHz
bandwidth, the driver amplifiers and predistortion linearizers are
required to have only a 6 MHz bandwidth in the arrangement of FIG.
2a, compared with a bandwidth of 120 MHz in the prior art
arrangement of FIG. 1a. It should be noted that the block
conversion reduces the center frequency but not the cumulative
bandwidth, so the percent bandwidth is increased by the block
conversion.
FIG. 2b is a simplified block diagram of a portion of the
arrangement of FIG. 2a. In particular, predistortion linearizer 36
may include a linearizer 46, which may be a conventional linearizer
such as is described in U.S. Pat. No. 5,015,965 issued May 14, 1991
in the name of Katz, et al. Any other conventional linearizer can
be used. As illustrated in FIG. 2b, linearizer 46 is bypassed by a
controllable switch 48. Those skilled in the art realize that while
switch 48 is illustrated by a mechanical switch symbol, switches
adapted for GHz frequency ranges must be used and may be electronic
rather than mechanical in nature. Further, in order to avoid
impedance perturbations, a switch such as 48 bypassing linearizer
46 may also include other switch portions intended for
disconnecting linearizer 46 from the line when bypass switch 48 is
closed.
FIG. 3 illustrates another possible arrangement according to the
invention. In FIG. 3, the cascade of a driver amplifier 34 and
final amplifier 38 is switched among channels by switch arrangement
23, on the theory that the amplifiers are more likely to fail than
the linearizer. This has the disadvantage that each driver
amplifier 34 must have a bandwidth equal to the cumulative
bandwidth of the V signals, but since each driver amplifier
operates at lower signal amplitude levels, the bandwidth is
achieved more readily than in the final amplifiers. Another
possible disadvantage is that the various predistortion equalizers
operate at signal levels which are less than in the arrangement of
FIG. 2a by the gain of the driver amplifier 34. If the driver
amplifiers have 20 dB of gain, the power level at which each PDL 36
operates is also reduced by 20 dB compared with FIG. 2a. Many of
the components of a linearizer, such as diodes, may exhibit less
control range at the lower signal levels.
Other embodiments of the invention will be apparent to those
skilled in the art. In particular, each linearizer 36 may be made
redundant, so that in the event that a linearizer fails, a
corresponding one (similarly optimized for frequency response in
the channel) can be substituted therefor. Also, post-distortion
linearizers may be used in the same fashion, by connecting the
post-distortion linearizer in a particular channel, as for example
channel 1, between output switch 28 and combiner 30. A satellite
may include more than one system such as that described, for
example one system receiving at 14 GHz and retransmitting at 12
GHz, with another system receiving at 6 GHz and retransmitting at 4
GHz. AGC or limiter amplifiers may be used as desired. While a
system has been described in which the nonlinearity arises from a
travelling-wave tube, the invention may be used with any broadband
nonlinear apparatus operating in a narrowband-channel context, and
particularly with solid state power amplifiers.
* * * * *